A108-06
Observed summertime patterns in coastal low cloud frequency for the Pacific Northwest, 1996-2017

Thursday, 10 December 2020: 20:50
Virtual
Alex W Dye, Oregon State University, Corvallis, OR, United States, Bharat Rastogi, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, United States, Rachel E. S. Clemesha, Scripps Institution of Oceanography, La Jolla, CA, United States, John B Kim, USDA Forest Service Pacific Northwest Research Station, Corvallis, OR, United States, Roger M Samelson, Oregon State Univ, Corvallis, OR, United States, Christopher J Still, Oregon State University, Forest Ecosystems and Society, Corvallis, OR, United States and A. Park Williams, Lamont -Doherty Earth Observatory, Palisades, NY, United States
Recorded Presentation
Abstract:
Summertime low clouds are common in the Pacific Northwest (PNW), but spatiotemporal patterns have not been characterized. We show the first maps of low cloudiness for the western PNW and North Pacific Ocean using a 22-year satellite-derived record of monthly mean low cloudiness frequency for May through September and supplemented by airport cloud base height observations. Domain-wide cloudiness peaks in midsummer and is strongest over the Pacific. Empirical orthogonal function (EOF) analysis identified four distinct PNW spatiotemporal modes: oceanic, terrestrial highlands, coastal, and northern coastal. There is a statistically significant trend over the 22-year record toward reduced low cloudiness in the terrestrial highlands mode, with strongest declines in May and June; however, this decline is not matched in the corresponding airport records. The coastal mode is partly constrained from moving inland by topographic relief and migrates southward in late summer, retaining higher late-season low cloud frequency than the other areas.
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